The present disclosure relates to a method and apparatus for counting fibers suspended in air or other gases in order to detect the presence of fibrous material, such as asbestos fiber, other natural and man-made fibers, and the like that may be harmful or injurious to health when inhaled into the lung. While automatic fiber counting devices have been developed (U.S. Pat. No. 4,940,327), there are major shortcomings in the instrument design that have limited their usefulness in the past. The present apparatus is aimed at overcoming the shortcomings of the previous fiber counting devices in order to have a reliable, high accuracy modern instrument for automatic fiber counting in the work place environment.